It is known that Greenland Eskimos rarely suffer from atherosclerotic cardiovascular diseases. This fact has been attributed to the consumption of high amounts of marine animal oil. The active ingredient in fish oil is (all-Z)-5,8,ll,l4,l7-eicosapentaenoic acid (hereinafter EPA). Most marine oils also contain docosahexaenoic acid (DHA). EPA and DHA are known to be precursors in the biosynthesis of prostaglandin E.sub.3 (PGE.sub.3) and thus provide at least come protection against cardiovascular diseases (see U.S. Pat. No. 4,097,602 and British patents 1,604,554 and 2,033,745).
Despite the fact that marine animal oil and the components thereof are known to be very healthful, people of the modern Western world very rarely consume marine animal oil, such as cod liver oil. Undoubtedly, one of the main reasons why there is not a greater consumption of marine animal oil, such as cod liver oil or other fish oils, is the particularly distasteful odor and taste which can best be described as repulsive. If marine animal oil could be deodorized and improved in taste, and particularly if other undesirable components could be removed therefrom, it is anticipated that the dietary uses of marine animal oil as an inexpensive source of highly polyunsaturated fat would increase greatly, and a corresponding improvement in public health would follow inevitably therefrom.
The poor flavor of raw, untreated marine animal oil is due to the presence of methylamines, i.e., trimethylamine, dimethylamine and oxides thereof, naturally present in fish, as well as peroxides and epoxides resulting from decomposition of the oil. The decomposition of marine animal oil occurs to a noticeable degree almost immediately after the slaughter of the fish. The addition of traditional antioxidants to the processed oil therefore does not rid the oil of its rancid odor and flavor.
Edible oils are typically deodorized by steam treatment and/or distillation. However, the necessary heating can deteriorate the oil. Furthermore, these conventional treatments do not remove trimethylamine or peroxides.
Past efforts have yielded various methods of masking trimethylamine odors. In one method (Chemical Abstracts 96:5061n), the fish odor is masked by cooking wine, cooking sake, mirin or synthetic sake, in decreasing order of effectiveness. The water-soluble organic acid components and amino acids of sakashio, a fermented seasoning typically used for masking the odor of trimethylamine, are known to mask trimethylamine odor, depending upon the pH of the solution in which they are present and the degree of buffering action they offer (Chemical Abstracts 97:108677t). Soy sauce exhibits a similar masking and buffering effect (Chemical Abstracts 91:156314v). Vinegar has also been used to suppress trimethylamine odor in fish and has been found to decrease the trimethylamine concentration in fish as the vinegar concentration is increased (Chemical Abstracts 91:191562w). A combination of an organic acid and a perfume has also been used to counteract trimethylamine odors (Japanese Kokai 56-160974).
Unfortunately, the odor of trimethylamine is only masked and not obviated by the above agents. These masking agents also add flavors to the fish, which, although they may not be disagreeable, are certainly undesirable if an odorless, tasteless product which may be further processed and used, for example as a cooking oil, is desired. Furthermore, these masking agents do not remove peroxides. Peroxides not only cause taste degradation, but are also mutagenic and possibly even carcinogenic.
Processes are known for removing trimethylamine from fish processing plant waste gas using activated sludge (Chemical Abstracts 94:89545t) or by passage through a zeolite tuff layer (Chemical Abstracts 82:102701g). The process conditions clearly render such processes unsuitable for treating oil for human consumption. Moreover, such processes are applicable to purification of gases rather than oils. Again, there is no reason to believe that such a process is applicable for the removal of peroxides.
Japanese patent 71-30195 discloses a process for removing peroxides from fish oils by the addition of metal oxides, such as Al.sub.2 O.sub.3, ZnO, SiO.sub.2 gel, and NiO, and subsequently reacting the oil with steam at 150-220.degree. C. and 5-6 mm Hg for 50-70 min. This process has several drawbacks. The necessity of reacting the oil with steam requires the consumption of a large amount of energy and the heat treatment over such a substantial period of time will ultimately degrade the oil and cause polymerization. The necessary use of vacuum further increases energy consumption.
In the U.S. Pat. No. 3,664,851, the deterioration of fatty acids is counteracted and slightly reversed to some extent by the addition of a compound of the structure RCH.dbd.CHCH.dbd.CH--Z where R is an alkyl group and Z is a polar group. This compound, however, has a flavor of its own and, if used as a counteractant in a highly deteriorated oil, must be added to the oil as a precursor which forms the compound at an appropriate rate to reverse deterioration without affecting the flavor.
It has been suggested in the prior art to prevent the deterioration of oil by the addition of a thiodipropionic acid or dialkyl thiodipropionate, such as dilauryl thiodipropionate (see, for example, Chemical Abstracts 70:69889; and U.S. Pat. Nos. 3,590,056; 3,573,936; 3,628,971; and 3,784,480). Usually, the dialkyl thiodipropionate is used in conjunction with a second antioxidant. The dialkyl thiodipropionate acts as a scavenger to prevent further oxidation of the oil. Unfortunately, dialkyl thiodipropionates have a highly objectionable odor due to release of sulfhydryl groups. Moreover, dialkyl thiodipropionates have only been used to prevent the deterioration of oils and have not been used to remove products of oil deterioration.
The use of antioxidants, and particularly the mixture of .gamma.-tocopherol and ascorbic acid, to preserve oils is also known (U.S. Pat. No. 4,101,673). However, as mentioned above, serious deterioration of marine animal oil usually occurs before antioxidants may be added. Thus, it is clear that the mere addition of antioxidant to oils according to known methods is insufficient for the production of an odorless oil from highly unstable marine animal oils.